Prediction of the Supply of Soil Nitrogen to Grass

Summary Predictions of the supply of soil nitrogen (N) to grass over a growing season requires an assessment of nitrate N plus ammonium N (1) present in the soil at the beginning of the season and (2) likely to be released from the potentially mineralizable organic N during the season. Although a number of methods have been proposed for predicting soil N supply, and in particular the component arising from the potentially mineralizable N, many are too time consuming for routine use, and others have not been proven effective over a range of soils. However, in recent studies, the measurement of nitrate N plus ammonium N, extracted either (1) by boiling with 1M or 2M KCl or (2) by shaking with 0.05M Ba(OH)₂, was found to account for 80% of the variation in the yield of N in the herbage of perennial ryegrass (Lolium perenne L.) grown in pots on 21 soils under uniform environment conditions. The prediction of soil N supply in the field, at over 18 sites in the UK, was substantially improved when the results of soil analysis were adjusted by a factor, calculated for each site, based on mean values of temperature and soil-water status.